The NESC-IV project addressed the transferability of fracture toughness data from laboratory specimens to applications that assess the integrity of reactor pressure vessels subjected to upset and normal loading transients. The main focus was six biaxial bend cruciform tests of surface-breaking semi-elliptic defects in a RPV longitudinal weld and four uniaxial tests on extended sub-clad defects in RPV plate material. The co-ordinated experimental/analytical program drew from major elements of the US Heavy Section Steel Technology Program. Extensive materials testing and fracture analyses were performed by 20 European organizations, co-ordinated by the Network for Evaluating Structural Components (NESC).